There is an ongoing need to improve fuel economy, reduce consumption of fuel, and reduce emissions in modern vehicles. Hybrid powertrain systems, including belt-alternator-starter (BAS) systems, electro-mechanical drive and electro-hydraulic drive systems are proposed to manage vehicle propulsion and minimize fuel consumption. One control scheme used in hybrid powertrain systems to improve fuel economy is commonly referred to as engine start/stop. Engine start/stop systems typically comprise deactivating or turning off the internal combustion engine under specific conditions during ongoing vehicle operation. Such specific conditions include idle periods at stop lights, and during stop-and-go traffic. Engine start/stop systems have delayed response times which limit their acceptability. Response time issues arise when an operator commands a stopped vehicle to proceed, through an action including depressing an accelerator pedal. At such time, the operator desires immediate responsiveness from the vehicle, vehicle launch may require that the engine be cranked and started prior to providing tractive torque to the vehicle wheels. The time to crank and start the engine can comprise several seconds. This operation is shown with regard to FIG. 2, wherein there is an extended period of time from when a lead vehicle increases speed from standstill until the operator detects such movement, releases the brake, manipulates an accelerator pedal, thus starting the engine and effecting forward vehicle motion.
Therefore, there is a need for an engine start/stop system for a vehicle powertrain which improves responsiveness, especially upon restarting of the engine, to better control forward motion of the vehicle during specific conditions defined by vehicle operating conditions, and traffic and road conditions.